1. Field of the Invention
The present invention relates to a semiconductor laser and a method for manufacturing the same; and more specifically to semiconductor laser intended to reduce the leakage current at a window layer and a method for manufacturing such a semiconductor laser.
2. Background Art
In recent years, broad-band optical communications have been progressed, and public communications networks using optical fibers have been widely used. With these trends, the transfer of a large quantity of information at low costs has been demanded. For these reasons, the quantity of information handled by information and communication systems has become enormous. Therefore, high speed of handling a large quantity of information at low costs and high reliability has been demanded for the communication systems.
One of major parts for information and communication systems is a semiconductor laser system. This system is required to be able to oscillate high-power laser beams efficiently at a low cost. As a high-speed large-capacity memory device, a DVD-R/RW device has been increasingly demanded recently. In the system, a high-power semiconductor laser (red laser of an emission wavelength of around 650 nm) is used. In the system, an AlGaInP/GaAs-based material having an ability of high-speed processing of information output at high efficiency is used, and such a material has been developed.
For example, in Japanese Unexamined Patent Publication No. 2003-31901, a structure of such a semiconductor laser is described. The above-described semiconductor laser is fabricated using an n-type GaAs substrate. An n-type AlGaAs buffer layer, an n-type AlGaInP first clad layer, an i-type AlGaInP first light-guide layer, an active layer having a multi-quantum well structure, an i-type AlGaInP second light-guide layer, a p-type AlGaInP second clad layer, a p-type GaInP etching stopper layer, a p-type AlGaInP third clad layer, a p-type GaInP band discontinuity reduction layer (BDR layer), and a p-type GaAs cap layer are sequentially stacked on the n-type GaAs substrate. By the third clad layer, the BDR layer, and the cap layer, a stripe ridge is formed. An n-type electrode is formed on the back face of the n-type GaAs substrate, and a p-type electrode is formed on the cap layer. The front end and rear end of an optical waveguide are formed so as to envelop the active layer, and a window layer, formed by disordering a part of the active layer by diffusing Zn, is formed in the vicinity thereof.
When the window layer is formed, Zn is diffused toward the n-type GaAs substrate, and a part of the active layer is disordered by Zn. Here, the Zn diffusion rate in buffer layer (AlGaAs) is lower than the rate in the first clad layer (AlGaInP). Therefore, the diffusion of Zn stops at the buffer layer to form p-n junction, and Zn is not diffused into the substrate side. Since AlGaAs has a large band gap energy, junction leakage produced in this layer can be held down.
In the conventional semiconductor laser described above, the diffusion of Zn must be stopped at the buffer layer (AlGaAs) in order to minimize the leakage current at the window layer. Therefore, a thick buffer layer had to be formed. When the buffer layer (AlGaAs) is formed, a crystal growing apparatus optimized for growing n-type AlGaInP layer. At this time, since an As-based material and a P-based material are used in this apparatus, if AlGaAs is grown to be thick, an exhaust gas filter is choked at a high frequency. This is significant when a material having a high Al composition is used. In addition, since the carbon intake of an AlGaAs material is large, the growing rate must be held low to avoid carbon intake. Therefore, it is difficult to raise growing rate, and the operation rate of the crystal growing apparatus is lowered. This problem is also marked since carbon intake increases when AlGaAs has a high Al composition. Then, the operation rate of the crystal growing apparatus is lowered, and manufacturing costs are elevated.